Tupac Shakur was killed more than 15 years ago — three years before the first Coachella Valley Music & Arts festival was held. But thanks to a trick of light, he's probably the single most talked about musician who performed at this year's version of the festival.

Except, well, "performed" is a funny word to use. The Tupac who appeared onstage during the headlining set by Snoop Dogg and Dr. Dre on Sunday was a hologram, more a feat of technology and bravado than a performance. (You can watch video, which includes a fair amount of profanity and other NSFW language, here.)

The image looked shockingly good, but how did it work? James Montgomery wrote about the Tupac hologram for MTV news, and explained to NPR's Audie Cornish that the Digital Domain Media Group, a company that has produced special effects for movies like X-Men: First Class, Transformers: Dark of the Moon and The Curious Case of Benjamin Button, used "really old theatrical technology" with a "2012 spin."

"There's an overhead projector that sort of reflects down onto basically a tilted piece of glass that's sort of on the stage floor," Montgomery says. "That then reflects the, well, reflection onto a mylar sort of screen, and it projects in this sort of 3-D kind of thing where it allows the other performers to sort of walk in front of Tupac and basically interact [with] him."

Montgomery says the exact technique behind the technology is still a little bit in the dark, so to speak. "You don't know whether or not they hired an actor to portray him and then sort of put digital clothing over this actor in post-production, or they built it in a computer," he says.

But the technology is there, and don't bet it'll be limited to this single appearance. Already, Snoop and Dre are reportedly thinking about taking the Tupac hologram out on tour. Montgomery thinks it won't end there.

"Once this becomes a little less cost prohibitive, given the wild popularity of deceased stars like Elvis or Michael Jackson, I can see Las Vegas shelling out a lot of money to have these sort of 'live reviews,' " he says. "It's also interesting if you look at the current stars of today, someone like Madonna or a Paul McCartney. Are they looking at what happened with Tupac, and are they thinking, maybe I have to rewrite my will and sort of include something that says, 'I don't want my likeness projected in 3-D holographic form at any point in the future.' "

Transcript

AUDIE CORNISH, HOST:

Technology took center stage at the Coachella Music Festival this past weekend. During a performance by hip-hop impresario Dr. Dre and rapper Snoop Dog, the stage went dark. A lush soundtrack kicked in with piano and an orchestra behind it and then, as if rising from the grave, Tupac Shakur appeared.

(SOUNDBITE OF COACHELLA PERFORMANCE FEATURING TUPAC SHAKUR HOLOGRAM)

Rather, a remarkable, convincing holographic projection of the late rapper; one that could sing, dance and greet the crowd.

(SOUNDBITE OF COACHELLA PERFORMANCE FEATURING TUPAC SHAKUR HOLOGRAM)

CORNISH: Tupac Shakur died more than 15 years ago. But armed with this new technology, Dr. Dre and Snoop Dog are reportedly looking into the logistics of taking the virtual Tupac on tour. To talk about what to make of this development, we have James Montgomery on the line. He's a staff writer for MTV News. Hi there, James.

JAMES MONTGOMERY: Hey, how are you?

CORNISH: So the company behind the rapper's - you know, ghostly appearance, for lack of a better term...

MONTGOMERY: Sure.

CORNISH: ...is called Digital Domain Media Group. And they're the guys behind, like, the technology for "The Curious Case of Benjamin Button," and stuff we see in movies.

MONTGOMERY: Right.

CORNISH: So what can you tell us about how this technology worked on a live stage?

MONTGOMERY: Well, the funny thing is, is this technology actually is based off, you know, really old theatrical tricks and techniques. You know, Pepper's ghost, basically using reflective glass and things like that, only in a very 2012 spin. There's an overhead projector that sort of reflects down onto, basically, a tilted piece of glass that's sort of on the stage floor. That then reflects the - well, reflection - onto a mylar sort of screen, and it sort of projects in this sort of 3-D kind of thing where it allows the other performers to sort of walk in front of Tupac and basically interact for him.

CORNISH: Interestingly, you have Tupac - this virtual Tupac - greeting the crowd. Coachella, you know, not audio that you have somewhere from...

MONTGOMERY: Sure, sure.

CORNISH: ...15 years ago. So how is this a game-changer in terms of the music industry? What's the potential here...

MONTGOMERY: Right. That's...

CORNISH: ...to tour?

MONTGOMERY: Right. That's the real interesting thing - is the fact that, you know, you don't know whether or not they hired an actor to portray him and then sort of basically put digital clothing over this actor in post-production, or they built it in a computer. But, also, you know, when Tupac appeared on the stage, he greeted the crowd. What's up, Coachella? Which is impossible, given that he died in 1996, and the first Coachella didn't happen until 1999.

So it raises some sort of interesting questions about, you know, what do we do with deceased stars, deceased celebrities, you know, going forward? The idea of putting recorded dialogue in, essentially, Tupac's dead body, for lack of a better term, is kind of troubling because who knows where we go from here? Who knows, next time he - you know, he's not saying something about Norelco razors or, you know, perhaps the idea of - once this becomes a little less cost-prohibitive, given the wild popularity of, you know, deceased stars like Elvis or Michael Jackson, I can see, you know, Las Vegas shelling out a lot of money to have these sort of quote, unquote, "live reviews."

It's also interesting if you look at some of the current stars of today, someone like Madonna or a Paul McCartney. Are they looking at what happened with Tupac and are they, you know, thinking, maybe I have to sort of rewrite my will and sort of include something that says, I don't want my likeness projected in 3-D holographic form at any point in the future.

CORNISH: I don't want to rain on the parade of this, but is there something distasteful here?

MONTGOMERY: You know, I do think there are some sort of issues with this. You know, look at someone like Kurt Cobain. You know, the control of his estate, Courtney Love being one of them, over the years, have done things like, you know, having him show up on a pair of shoes or, a few years ago, Kurt Cobain showed up as a playable character in a rock band video game.

So if you are a fan of music and care about things like legacy of artists, we could be heading down a slippery slope here. In most cases, family members, you know, close friends control the estates of these dead stars. But who's to say if they were to sell them to a business interest? Something like James Dean - you know, he was sold to a business interest now. And there's these companies that will license the likenesses.

And in the past, there's been still images, you know, maybe recorded music. But who's to say we couldn't have a hologram James Dean or a hologram Kurt Cobain onstage in Las Vegas performing the hits? So it is very interesting, to say the least.